Regulation of Cell Cycle Length in Progenitor Cells of the Developing Vertebrate Retina

Abstract

Progenitor cells exist in two distinct phases during development with different forms of division: preneurogenic and neurogenic. Preneurogenic progenitor cells produce two cells that divide again while neurogenic progenitor cells divide to produce one or both cells that withdraw from the mitotic cycle and differentiate. Preneurogenic and neurogenic cells are fundamentally different in that only preneurogenic progenitor cells express the Notch ligand, Delta 1, and only neurogenic progenitor cells express the bHLH transcription factor, E2A (Yang et. al., 2009). Additional differences in these cell types are not well understood. The vertebrate retina is an effective model system for studying these differences in progenitor cells because it is a part of the central nervous system that buds off from the brain during early development. As the retina develops, it matures from the center to the periphery with differentiated cells towards the center and progenitor cells replicating without differentiation in the periphery (Dutting et al., 1983). The neurogenic front marks the border between preneurogenic and neurogenic progenitor cells. Ganglion cells are the first postmitotic, differentiated cells to be generated at the neurogenic front, which is present for several days in chick embryo retina. It is well documented that as development progresses, cell cycle length increases (Takahashi et. al. 1995). It is unknown whether this is a gradual increase throughout development or if it is related to a change in cell type. In this study we explore the question, is the change in progenitor cell mode from preneurogenic to neurogenic accompanied by a change in cell cycle length? The retina will be used to answer this question because preneurogenic and neurogenic progenitor cells can be studied in the same animal during several days of early development. To determine if there is a difference in cell cycle length between these differentiated, neurogenic progenitor cells in the central retina and the undifferentiated, preneurogenic progenitor cells in the periphery during development, we used two BrdU injections. These thymidine analogs are incorporated into the cell’s DNA during S phase of the cell cycle and we detected them using immunohistochemistry. Immunohistochemistry with RA4 was also used to determine the location of the most peripheral ganglion cell, which marks the neurogenic front. The total number of cells and the number identified by each BrdU injection were counted in both preneurogenic and neurogenic retina. We used an equation considering the percentage of cells that incorporated only one of the thymidine analogs and the survival times after each injection to determine the total cell cycle length. Here we show that the cell cycle of neurogenic progenitor cells is longer than that of preneurogenic progenitor cells, indicating that this is a fundamental difference in the cell types and not due to a gradual increase in cell cycle length throughout development.